The invention relates to a method and device for producing an unusually efficient flow in those portions of turbo machines downstream of blading sections, with particular application to gas turbine and jet engine compressor outlets and turbine exhaust outlets.
Turbo machinery is becoming more widely applied to new and different applications as their performance improves with the utilization of new materials and better design analysis methods. For example, gas turbines and jet engines are becoming more powerful, more compact, and lighter, thereby having broader uses than ever before.
Turbo machinery efficiency depends on both achieving higher turbine inlet temperatures and on reducing various mechanical and flow losses. The flow losses are particularly large for flow in diverging sections of ducts, which are found in most gas turbines and jet engines downstream of the compressor and downstream of the turbine. In these ducts, the flow is intended to expand in area and decelerate, exchanging kinetic energy for pressure energy. Typically, only 40 to 60 percent of the kinetic energy is recovered to become useful pressure energy. The remainder is converted either to heat, mostly by friction within the wall flow boundary layer, or exits the expanding area duct as unrecovered kinetic energy to become heat in a collector or receiver volume. However, the amount of area expansion practical, and therefore pressure recovery, is severely limited by flow separations or aerodynamic stalls that may develop if the expansion exceeds an area ratio of about 1.7 to 1, and will often develop at an area ratio of 2 to 1 unless the duct wall total divergence angle is kept small, usually below about 8 degrees. These small divergence angles mean that the expanding area duct will be long, however, and will not be compact or light. Even a tendency of momentary stalls or roughness, often of no concern if only efficiency is considered, will possibly result in more noise and vibration, an increase in compressor outlet pressure and a resultant possibility of aerodynamic stall of the compressor, which can be quite destructive. Accordingly, an expansion ratio of 2:1 or less is accepted practice for most turbo machines.
Because these blading outlet losses may total two percent of the compressor power input, or three percent of the turbine power output, these losses significantly affect fuel economy and power. In an industry where a performance difference of several percent in fuel economy is important, a 2 to 5 percent improvement is very significant, particularly for airline and electric power generation users who purchase enormous quantities of fuel.
Two specific examples of turbo machinery, a gas turbine exhaust outlet with both a divergent duct and a bend, and a divergent compressor outlet that may include a bend are discussed below.
Gas turbine engines are used in a variety of applications for the production of shaft power. In most gas turbine installations the turbine exhaust vents into an enclosure, often called a receiver or collector box, which is used to collect flow, then to direct the exhaust flow away from the axis of the turbine system. The typical gas turbine collector box is an enclosure which surrounds the outlet end of the turbine tailpipe and collects the exhaust gas to direct it away from the gas turbine tailpipe. Most often, the tailpipe is a divergent duct, such as a cone. Most collector boxes turn the exhaust gas 90 degrees from the gas turbine centerline, although exhaust paths from zero degrees to 160 degrees from the gas turbine centerline are used.
In small gas turbines, the collector box typically has a large width in relation to the diameter of the turbine last stage. The size of most collector boxes, however, does not increase proportionately with gas turbine capacity due to constraints such as maximum shipping dimensions, cost, or available installation space.
As the relative size of the collector box decreases with respect to the turbine outlet diameter, gas velocities in the collector box increase. Any turbulence in the collector box is therefore likely to cause large velocity differentials within the collector box as well as in the downstream ducts. These velocity differentials may induce destructive vibrations in the turbine, collector box or downstream ducts. The velocity differentials may also create steady or transient flow reversals or stalls in the exhaust gas flow which can increase vibrations levels, overall noise levels, and system back pressure. An increase in system back pressure will lower the turbine efficiency.
The turbine tailpipe typically protrudes into the collector box from the turbine outlet. The tailpipe may be either straight or divergent (usually conical) and is often called a "tailcone". Because it maintains high exhaust gas velocities, the straight (non-expanding area) tailpipe design is less likely to experience stalls or flow reversals in the tailpipe. The straight design, however, maintains high back pressure which reduces the overall engine efficiency. The divergent tailpipe design slows the flow in a diffuser effect, exchanging kinetic energy for pressure, which improves engine performance. This exhaust flow expansion, however, also increases the risk of aerodynamic stalls or flow pattern switching in the tailpipe which can cause destructive vibrations forces and noise.
There are two ways to extract output shaft power from a gas turbine. The first is route the power output shaft through the engine and out the compressor end. This design allows a clean collector box interior which contains only the exit of the tailpipe, but no shaft. The second design, which is found more often in industrial turbines, has the output shaft passing through the exhaust collector box. Depending on the power shaft coupling and turbine rear bearing cooling design, the power output shaft housing may be small or large in relation to the size of the collector box. In large gas turbines where the collector box size is restricted for shipping, cost, or other reasons, the power output shaft housing can occupy a large percentage of the available volume of the collector box which in turn increases local velocities in some areas and blocks exhaust gas in others. This arrangement may increase the velocity differentials in the collector box, promote destructive vibrational and acoustical forces, and increase back pressure.
Prior to the invention disclosed below, the most efficient collector box designs utilized large volume, divergent conical tailpipes, and in the case of gas turbines with power output shafts in the collector box, divergent power output shaft housings. These collector boxes are found in smaller or mid-range gas turbines where the collector box can be large in relation to the last stage of turbine diameter so the maximum tailpipe outlet exhaust velocities can be reduced, thereby lowering the differential exhaust velocities within the collector box and making any stalls or turbulence less likely to cause destructive vibration. This design also recovers spin energy, if any, in the exhaust flow.
For a few turbines the most efficient collector box designs have radial turning vanes to straighten the spinning flow in the tailpipe. However, these radial vanes may result in tailpipe stalls when the tailpipe is divergent. This design is typically found in smaller units, particularly those with a radial turbine element in the power turbine.
For reference, in all succeeding discussions, the turbine axis is deemed horizontal and the exhaust outlet is upward. One prior art approach for improving turbine exhaust collector box flow efficiency is to install a streamlined fairing on the bottom and top of the power output shaft housing to streamline the flow over the housing, sometimes in combination with conventional turning vanes in a rack. (The bottom is the side away from the collector box exit.) This system is effective when the power output shaft housing has a small diameter in relation to the width of the collector box, but is not used for practicality and cost reasons. In larger turbines, where the collector box is relatively smaller compared to the shaft housing, the fairings have been shown to be far less-effective and are generally ineffective.
Another approach to improving collector box flow efficiency is to add turning vanes, of various designs but usually ring-shaped and in a rack, to improve the flow distribution inside the tailcone and collector box. These have been partially successful where the collector box has large size compared to the last stage turbine outlet. However, they do not solve the specific problem of stalls in all the identified problem areas. They also are under high mechanical stress, constant vibration, and thermal stresses which can cause them to fail, sometimes over a short period of time. Successful turning vanes are expensive, but still allow large scale turbulence that often causes noise and destruction of wall insulation and coverings.
To reduce roughness and flow separations in the divergent engine tailpipe, obstructions and fillers have been installed in the lower half of the tailpipe (on the side opposite the collector box exit) to increase the flow velocity in this area. This velocity increase reduces the probability of stall formation in the tailpipe. Although this arrangement improves flow stability, the increased velocity also reduces the expansion effects of the tailpipe and thereby reduces the pressure and power recovery compared to a stall-free exhaust expansion. Also, smaller transient stalls or roughnesses may still form in the tailcone or collector box, and there is relatively high velocity collector box turbulence, which indicates that the basic problem has not been completely solved.
In most turbo machines, including radial, axial, and mixed flow compressors, the compressor section ends in a duct of expanding area, most often of generally annular shape for axial flows and of axially divergent shape for mixed or radial flows.
In both cases, there also may be one or more bends. Some radial or mixed flow compressors also include a volute shape. This duct of expanding area decelerates flow, converting some kinetic energy to pressure energy. Sources of flow losses are as discussed previously.
The typical 1 to 1.8 expansion ratio duct would, by previous technology, terminate in a receiving volume that also contains the fuel combustion can. The addition of a bypass passage leading from each side of the expansion duct near its outlet and downstream of struts and releasing flow into the tail end of the combustor and into the turbine area where it rejoins the main flow allows the inlet duct expansion ration to be increased to 2.5 to 1 or 3.5 to 1 with excellent stability and flow smoothness. In terms of efficiency, improvements will vary from one turbine to another, but 1.0 to 4 percent compressor efficiency improvements are estimated.